Four years ago, John Kerry flirted with the idea of making John McCain his running mate. Today, he is denouncing the Arizona Senator for “a stunning failure of leadership,” and running a nasty, hate-filled campaign.

In a letter to supporters, the Massachusetts Democrat — no stranger to smears himself — ramps up his criticisms of McCain to new heights. In addition to airing disgust with the tone of the McCain crowds, he rips Gov. Sarah Palin for making “outrageous charges that only a few years ago would have disqualified someone from serious consideration for national office.”

The letter reads:

John McCain has shown a stunning failure of leadership. His campaign, in a time of economic crisis and foreign policy drift, has degenerated into a negative and nasty campaign of smears.

The reports are piling up of ugliness at the campaign rallies of John McCain and Sarah Palin. Audience members hurl insults and racial epithets, call out “Kill Him!” and “Off With His Head,” and yell “treason” when Senator Obama’s name is mentioned. I strongly condemn language like this which can only be described as hate-filled.

According to reports, every ad paid for by the John McCain campaign is now a negative ad — every single one! McCain allows his running mate to make outrageous charges that only a few years ago would have disqualified someone from serious consideration for national office.

We cannot stand by and allow this to happen. We need to fight back, spread the word about what kind of low campaign he’s running, and make sure people know the truth.

His strained relationship with McCain serves as a reminder of how much the political dynamics have changed in the past four years. It also begins to raise the question: what kind of reception will McCain receive either if he goes back to the Senate as a campaign loser or has to work with Congress as the next president?